Method for the purification of melamine-containing ammonia

ABSTRACT

The invention relates to a method for the purification of melamine-containing gaseous NH 3 , by means of partial condensation, whereby pure NH 3  gas and liquid NH 3  enriched with melamine are obtained.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a National Phase Patent Application ofInternational Application Number PCT/EP01/12691, filed on Nov. 2, 2001,which claims priority of Austrian Patent Application Number A 1888/2000,filed Nov. 8, 2000.

FIELD OF THE INVENTION

[0002] The application relates to a process for purifying gaseousmelamine-containing ammonia. The inventive process is suitable inparticular for purifying ammonia gases originating from high-pressuremelamine plants using anhydrous melamine workup.

BACKGROUND

[0003] In high-pressure processes for preparing melamine, urea isconverted to melamine in an endothermic liquid-phase reaction. Afterremoval of NH₃ and CO₂, a melamine melt under high pressure is obtainedwhich is then solidified in what are called wet workup processes byquenching with water. In what are called the dry processes, asdescribed, for example, in U.S. Pat. No. 4,565,867, WO 95/01345, WO97/20826 and WO 99/38852, the melamine melt is solidified by quenchingwith ammonia, by expanding the NH₃-saturated melamine melt or amelamine/NH₃ suspension at a temperature just above the melting point ofmelamine, by sublimation with subsequent desublimation or by cooling ina fluidized bed. However, in these processes, sometimes very largeamounts of gaseous NH₃ are produced which, depending on pressure andtemperature of the gas produced, comprise greater or lesser amounts ofmelamine. This already CO₂-free gas can be returned to the melamineplant only in part and sometimes only with difficulties. The melaminepresent in the NH₃ gas causes difficulties and faults, in particular inthe compressors, if the NH₃ gas needs to be compressed forliquification. A need therefore exists to free melamine-containing NH₃gas from melamine in a simple process.

SUMMARY OF THE INVENTION

[0004] The invention is directed to a method comprising partlycondensing the melamine-containing gaseous NH₃. This produces firstlymelamine-containing liquid NH₃ and, secondly, purified melamine-free NH₃gas which is separated off from liquid NH₃.

[0005] More particularly the invention relates to a process forpurifying melamine-containing gaseous NH₃, which comprises partlycondensing melamine-containing gaseous NH₃ by cooling, the melamineaccumulating in the liquid NH₃ and the purified NH₃ gas being taken off.

DESCRIPTION OF THE DRAWING

[0006] A more complete appreciation of the invention, and many of theattendant advantages thereof, will be readily apparent as the samebecomes better understood by reference to the following detaileddescription when considered in conjunction with the accompanyingdrawing, wherein:

[0007]FIG. 1 is a schematic view of an exemplary plant in accordancewith the invention.

DETAILED DESCRIPTION

[0008] The melamine-containing gaseous NH₃ can be cooled using anycooling device, for example in a cooling column or by means of liquidNH₃, liquid NH₃ being mixed with the NH3 to be purified, for example byspraying liquid NH₃ into the NH₃ to be purified or by passing the NH₃ tobe purified through liquid NH₃. Preferably, the melamine-containinggaseous NH₃ is passed through liquid NH₃, in which case it cools andpartly condenses. The resultant purified NH₃ gas is taken off and can ifappropriate be condensed, and the melamine-enriched liquid NH₃ isdischarged. The amount of liquid NH₃ discharged is preferably replacedby the condensing NH₃ gas, but it can also be replaced by feeding freshNH₃.

[0009] The melamine-containing NH₃ gas to be purified is alreadyCO₂-free and preferably originates from dry melamine processes as setforth above, for instance from processes for solidifying liquid orgaseous melamine under NH₃ pressure or a melamine/NH₃ suspension usingliquid supercritical or gaseous NH₃. In the dry melamine processes thesolidification is performed, for example, by quenching with NH₃, withNH₃ and liquid or gaseous melamine being sprayed into a quencher. In anexemplary embodiment, the melamine-containing NH₃ gas to be purifiedoriginates from the fluidized bed of a plant for solidifying melamine,into which the liquid or gaseous melamine is introduced into a fluidizedbed made up of solid melamine, or solid inert particles and solidmelamine, and maintained using NH₃ gas.

[0010] Preferably, the process is carried out continuously. The gaseousmelamine-containing NH₃ is, for example, partly condensed using an air-or water-cooled cooler or via direct contact with liquid NH₃.Preferably, the gaseous, melamine-forming NH₃ here is passed throughliquid NH₃, with melamine precipitating or dissolving and accumulatingin the liquid NH₃. Depending on from which plant for the dry melamineworkup the gas to be purified comes, the gas can be of varying pressureand varying temperature and, as a function thereof, differing melaminecontent. The lower limit of temperature is preferably just above therespective condensation temperature at the respective pressure, and theupper limit at the operating temperatures of the plant section fromwhich the NH₃ to be purified originates.

[0011] If the melamine-containing NH₃ originates, for example, from aprocess for solidifying gaseous melamine using, or in the presence of,ammonia, the pressure is, for example, about 1-20 barand about 1.5-15bar, respectively, and the temperature is, for example, about 290-520°C. If the melamine-containing NH₃ originates, for example, from aprocess for solidifying liquid melamine, lower temperatures down to roomtemperature are also possible, preferably from about 100° C. to justbelow the pressure-dependent melting point of melamine, for example fromabout 100 to 340° C., more preferably from about 200 to 320° C.

[0012] Higher pressures are also possible up to about 500 bar,preferably from about 5 to 250 bar, more preferably from about 10 to 100bar.

[0013] Pressure and temperature in the partial condensation must bechosen in such a manner here that the NH₃ can be condensed using thecoolant available. For example, when water is used as the coolant, thepressure shall not be less than 10 bar, in accordance with the ammoniacondensation curve.

[0014] The process can be preferably and particularly simply carried outin such a manner that the melamine-containing gaseous NH₃ is passedthrough liquid NH₃ and partly condensed. The NH₃ gas thus purified canthen be taken off and the melamine-enriched liquid NH₃ can bedischarged. The amount of melamine-containing liquid NH₃ discharged iscontinuously replaced by condensing NH₃. However, it is also possible tofeed in part fresh liquid NH₃. The partial condensation of the gaseousNH₃ can take place either directly on its introduction into the liquidNH₃, or else by condensation of the purified NH₃ continuously forming,for example using a downstream cooler.

[0015] On passing the NH₃ to be purified through the liquid NH₃,melamine-enriched liquid NH₃ is formed which, depending on pressure andtemperature, contains greater or lesser amounts of melamine. It can besaturated with melamine or subsaturated, or else solid melamine canalready have formed;in other words, melamine-containing liquid NH₃ is tobe taken to mean not only a solution, but also a suspension, of melaminein NH₃.

[0016] In a continuous process in which the melamine-enriched NH₃ iscontinuously discharged via an overflow, the melamine-enriched liquidNH₃ is customarily recirculated to a suitable section of the melaminepreparation process. This can be the reactor or a subsequent separator.Advantageously, the recirculation is into a device in which liquidmelamine is allowed to stand under ammonia pressure (“aging”) or into adevice in which already-solidified melamine is allowed to stand underammonia pressure (“tempering”).

[0017] Preferably, the melamine-containing liquid NH₃ is recirculated toa plant in which liquid melamine is quenched (solidified) underexpansion using liquid ammonia or the gaseous ammonia formed onexpansion of the liquid ammonia. Preference is likewise given torecirculating the melamine-containing liquid NH₃ to a plant in which amixture of gaseous NH₃ and gaseous melamine is quenched and solidifiedusing ammonia.

[0018] In another embodiment, the melamine-containing liquid NH₃ isrecirculated to a fluidized bed that comprises solid melamine, or solidmelamine and solid inert matter, and is maintained by NH₃ gas.

[0019] A possible plant for carrying out the inventive process is showndiagrammatically in FIG. 1. In the figure are shown the following:

[0020] (1) fluidized bed,

[0021] (2) pressure vessel,

[0022] (3) cooler,

[0023] (4) intermediate vessel,

[0024] (5) pump,

[0025] (6) melamine-enriched liquid NH₃,

[0026] (7) gaseous melamine-free NH₃,

[0027] (8) gaseous melamine-containing NH₃.

EXAMPLE 1

[0028] A melamine-containing gaseous NH₃ stream 8 of 424 kg/h comingfrom the fluidized bed 1 of a melamine solidification plant is passed,at a temperature of 280° C. and a pressure of 10 bar, from the bottomthrough the liquid NH₃ in a pressure vessel 2 equipped with a cooler 3which is filled with liquid NH₃. A portion of the NH₃ condenses and themelamine separates out in the liquid NH₃ 6. To maintain a constantliquid level in the pressure vessel, 384 kg/h of liquidmelamine-containing NH₃ 6 are discharged and recirculated to themelamine plant. The purified NH₃ gas 7 vaporizing in the pressure vesselpartly condenses in the attached cooler, and the non-condensed portionis taken off as pure NH₃ gas 7 at a rate of 40 kg/h.

EXAMPLES 2-4

[0029] In a similar manner to example 1, melamine-containing NH₃ gas waspurified at various pressures and temperatures.

[0030] The values for Examples 1 to 3 are listed in Table 1. TABLE 1 NH₃to be Temp. Pressure purified Recirculated Pure NH₃ Example (° C.) (bar)(kg/h) NH₃ (kg/h) (kg/h) 1 230 10 424 384 40 2 280 40 740 650 87 3 34010 380 340 40

1. A process for purifying melamine-containing gaseous NH₃, whichcomprises partly condensing melamine-containing gaseous NH₃, themelamine accumulating in the liquid NH₃/and the purified NH₃ gas beingtaken off.
 2. The process as claimed in claim 1, wherein themelamine-containing gaseous NH₃ is passed through liquid NH₃, beingcooled and partly condensed, the purified NH₃ gas is taken off andmelamine-enriched liquid NH₃ is discharged.
 3. The process as claimed inclaim 2, wherein the melamine-enriched liquid NH₃ contains the melaminein dissolved form.
 4. The process as claimed in claim 2, wherein themelamine-enriched liquid NH₃ contains solid melamine.
 5. The process asclaimed in claim 2, wherein the melamine-enriched liquid NH₃ isrecirculated to a plant for solidifying liquid or gaseous melamine. 6.The process as claimed in claim 2, wherein the melamine-enriched liquidNH₃ is recirculated to a fluidized bed which consists of solid melamine,or solid melamine and solid inert matter, and is maintained using NH₃gas.
 7. The process as claimed in claim 1, wherein themelamine-containing NH₃ gas originates from a process for solidifyingliquid melamine under NH₃ pressure using liquid, supercritical orgaseous ammonia.
 8. The process as claimed in claim 1, wherein themelamine-containing NH₃ gas originates from a process for solidifyinggaseous melamine under NH₃ pressure using liquid, supercritical orgaseous NH₃.
 9. The process as claimed in claim 1, wherein themelamine-containing NH₃ gas originates from the fluidized bed of a plantfor solidifying liquid melamine.
 10. The process as claimed in claim 1,wherein the melamine-containing NH₃ gas originates from a fluidized bedof a plant for solidifying gaseous melamine.